1. Field of the Invention
The present invention relates to sensing circuits and, particularly, to circuits for sensing audio input/output plugs.
2. Description of the Related Art
Portable electronic devices, such as cellular telephones and portable radios, are well known. It is further known to provide electronic devices having combined telephone and AM/FM radio functionality. Once clipped into position onto a user""s person (such as via a belt clip or armband), such devices may be used with greater freedom by way of attachable headpieces. For example, in the case of a cellular telephone, a headset may include a pair of earphones with a boom microphone extending towards the user""s mouth. Such headsets usually provide a channel for the microphone and a channel for mono- or bi-naural audio output to the speakers. Similarly, stereo headphones, providing left and right stereo channels, for use with portable AM/FM radios are well known.
Telephone headsets and stereo headphones are typically designed mechanically to use the same 2.5 millimeter two-channel plug and jack. However, the telephone headset and the stereo headphones require different electrical signal and pin configurations. Thus, in order to provide both AM/FM radio and cellular telephone functionality, an electronic device must generally be provided with one 2.5 millimeter jack for receiving the stereo headphone plug and another for receiving the telephone headset plug. Such devices may further include circuitry for sensing the presence or absence of a single plug. However, no jacks are known for sensing two different plugs.
Accordingly, there is a need for a single jack which can detect the presence of either a telephone headset or stereo headphones and provide either telephone functionality or radio functionality in response to their detection.
These problems in the prior art are overcome in large part by a headset/headphone sensing jack according to an embodiment of the present invention. A two-channel sensing circuit is provided which detects whether a headphone or headset is installed. By employing an impedance sensing circuit, it is possible to use a single common 2.5 mm jack for both radio and telephony operation. Formerly, two separate jacks would be required for both functions.
A sensing circuit is provided which includes one or more comparators for detecting whether a headphone or headset is installed. The comparator compares impedance levels to a predetermined reference taking advantage of the two different impedances of the devices. More particularly, if a headpiece is installed a low impedance is detected; when a headpiece is not installed a high impedance is detected. Typically, a sensing circuit is provided for two channels (e.g., left and right stereo, or microphone and audio output). If both channels are at low impedance, then the stereo headphones are installed. When only one channel is at low impedance, then the other channel is the microphone input and a telephone headset is installed. If neither channel is at low impedance, then nothing is installed. An electronic device according to the present invention is further capable of switching between AM/FM radio and telephone modes depending on whether a particular headpiece is installed.